The present invention relates to semi-rotary die cutters and printing presses of the type in which a removable thin die or printing plate is secured to a rotary cylinder.
Semi-rotary die cutters and printing presses are well known in the art and provide a continuously rotating cylinder for repetitive cutting or printing of successive sheets or continuous strip stock. In both applications it is common practise to provide a thin flexible plate which carries the die or print impression on the outer surface thereof and which wraps partially about the rotary cylinder. The die plate or flexographic printing plate is readily removable from the rotary cylinder to allow replacement. Various means have been proposed for releasably securing the plate to the rotary cylinder such as, for instance, vacuum, adhesives, mechanical fasteners and magnetic force.
In a semi-rotary unit sheets or continuous stock strip pass through the unit with a reciprocating or ‘Pilgrim step’ motion as the rotary cylinder rotates. The rotary cylinder supports over part of its circumference a printing plate or die cutter. The web in such a unit accelerates from standstill to the cutting speed which is the circumferential speed of the rotary cylinder, and then maintains a constant speed while the die knives or impression plate is in contact with the web. When the rotary cylinder has rotated to a position where the die knives or impression plate is no longer in contact, the web decelerates to standstill reverses direction and accelerates and finally decelerates back to standstill. The motion of the web through the unit is such as to ensure that the leading edge of the next repeat is always in the correct start position relative to the rotary cylinder.
For the die plate or flexographic printing plate to continuously match the repeat on the web the speeds of the rotary cylinder and of the web must be exactly synchronised. To this end, each plate is manufactured for use on a rotary cylinder of a particular, known diameter. If the plate is used on a rotary cylinder of a different diameter the repeat length will be altered. This difference may only amount to a few tenths of millimeters, but is enough to affect the quality of the finished product.
If a die or flexographic printing plate is used on a unit having a rotary cylinder with a circumference different to that of the unit for which it was originally intended the print or cut length will not match the web repeat as it should. To overcome this problem either the rotary cylinder must be changed to match the plate or a new plate must be made to match the circumference of the existing rotary cylinder. Neither solution is wholly acceptable as they both involve additional expense on additional cylinders and/or plates and considerable expertise in setting up the machine and bringing it into register on each changeover.